Screening

manish264@gmail.com

Manish Chaudhary

BPH(IOM), MPH(BPKIHS)

Screening

• The process, by which unrecognized disease or defects are identified by test that can be applied rapidly on a large scale is known as screening.

• Screening sorts out healthy people from those whom may have a disease.

•  It is not diagnostic.

Screening

• The early detection of–disease–precursors of disease–susceptibility to disease

in individuals who do not show any signs of disease

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Screening

• It is preventive function

• Detection of hidden disease or condition

• It differ from periodic health examination

• A screening test is not intended to be a diagnostic test

• Should be capable of wide application

• Inexpensive

• Can be used by health workers

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Screening

• Concept lead time

• Onset S point Diagnosis Usual Dx Outcome

Diagnosis = Screening Screening tests can also often be used as

diagnostic tests Diagnosis involves confirmation of presence or

absence of disease in someone suspected of or at risk for disease

Screening is generally in done among individuals who are not suspected of having disease

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Screening

• There is different between screening and diagnostic test:

• Health VS sick

• Group VS individual

• Result final VS require further test

• Criteria and cut off point VS overall evaluation

• Less accurate VS more accurate

• Not the basis for treatment

Screening and Diagnostic Tests Contrasted

Screening test Diagnostic test

1. Done on apparently healthy Done on those with indication or sick

2. Applied to groups (single disease considered)

Applied to single patient (All diseases are considered)

3. Test results are arbitrary and final Diagnosis not final, is the sum of all evidence

4. Based on one criterion or cut-off point (e.g. diabetes)

Number of symptoms, signs and laboratory findings.

5. Less accurate More accurate

6. Less expensive More expensive

7. Not a basis for treatment Used as a basis for treatment

Susceptible Host

Subclinical Disease

Clinical Disease

Stage of Recovery, Disability, or Death

Point of Exposure

Screening

Onset of symptoms

Diagnosis sought

Natural History of Disease

Detectable subclinical disease

Screening Process

TestNegative

Re-screen

Unaffected

Intervene

Affected

TestPositive

Population(or target group)

Screening

ClinicalExam

Population

Test -ve Test +ve

Re-screen

UnaffectedAffected

Re-Screen Screen Intervention

Screening test

Diagnostic procedure

Concept of Screening Process

Examples of Screening Tests

• Questions• Clinical Examinations• Laboratory Tests• Genetic Tests• X-rays

Aspect Requirement

Disorder Well defined

Prevalence Known

Natural history Medical important disorder for which there is an effective remedy available

Financial Cost-effective

Facilities Available or easily installed

Ethical Produce following a positive result generally agreed and acceptable

Test Simple and safe

Test performance Values in affected and unaffected individuals known, overlapping sufficiently small, and suitable cut off values defined

Basic Features of Screening Program

Uses of Screening a) Case Detection: This is also known as “ prescriptive

screening. It is defined as the presumptive identification of unrecognized disease, which does not arise from a patient’s request, e.g. neonatal screening. People are screened primarily for their own benefit.

b) Control of Disease: This is also known as prospective screening. People are examined for the benefit of other, e.g. screening of immigrants from infectious disease such as TB and Syphilis to protect the home population.

The screening program may, by leading to early diagnosis permit more effective treatment and reduce the spread of infectious disease and/or mortality from the disease.

c) Research Purpose: There are many chronic diseases whose natural history is not fully known ( e.g., cancer, hypertension). Screening may aid in obtaining more basic knowledge about the natural history of such diseases.

d) Educational opportunities: Screening program provide opportunities for creating public awareness and educating health professionals.

Types of screening• Mass screening:

Mass screening means screening of whole population or sub population as for example all adults. It is offered to all , irrespective of the particular risk individual may run of contracting the disease in question( e.g. tuberculosis)

• High risk or selective screening

Screening will be productive if applied selectively to high risk groups , the groups defined on the basis of epidemiological research. For example , since cancer cervix tends to occur relatively less in upper social groups, screening for cancer cervix in lower social class would increase new cases.

• Multiphasic screening

It has been defined as the application of two or more screening tests in combination to a large number of people at one time than to carry out separate screening tests for single disease.

• Case finding or opportunistic screening

It is restricted to patients who consult a health practioner for some other purposes.

Criteria of Screening Programme

The criteria for the screening Programme are based on two considerations:

• Disease to be screened and • Tests to be applied.

The Disease Criteria• The condition sought should be an

important health problem.• There should be a recognizable latent or

early asymptomatic stage• The natural history of the disease should be

clearly understood• There should be a test that can detect the

disease prior to the onset of sign and symptoms.

The Disease Criteria• Facilities should be available for the

confirmation of diagnosis• There should be an effective treatment.• There should be good evidence that early

detection and treatment reduces morbidity and mortality.

• The expected benefit of early detection exceeds the risks and costs.

Criteria of the Screening Test:The test must satisfy the criteria of• acceptability, • repeatability and • validity, • simplicity, • safety, • rapidity, • ease of administration and cost.

Validity (Accuracy)

• The term validity refers to what extent the test accurately measures which it purports to measure.

• Validity express the ability of a test to separate or distinguish those who have the disease from those who do not.

• For example Glycosuria is a useful screening test for diabetes, but a more valid or accurate test is the glucose tolerance test.

• Accuracy refers to the closeness with which measured values agree with “ true” values.

Two Components of Validity

Sensitivity and Specificity• Both these components should be

considered when assessing the accuracy of the diagnostic test.

• Expressed in percentage • Both are usually determined by

applying the test to one group of persons having the disease, and to a reference group not having the disease.

Screening Test Results by Diagnosis

Screening test results Diagnosis Total

Diseased Not diseased

Positive a (True positive) b (False positive) a+b

Negative c (False negative) d (True negative) c+d

Total a+c b+d a+b+c+d

“a” denotes those individuals found positive on the test who have the condition or disorder being studied (i.e., true positives).

“b” includes those who have the positive test result but who do not have the disease (i.e., false positives).

“c” includes those with negative test result who have the disease (i.e., false negative)Finally, those with negative results who do not have the disease are included in group “d” (i.e. true negatives).

The following measures are used to evaluate a screening test:

• Sensitivity = a/(a+c) x 100• Specificity = d/(b+d) x 100• Predictive Value of a positive test = a/(a+b) x 100• Predictive Value of a negative test = d/(c+d) x 100• Percentage of false negatives = c/(a+c) x 100• Percentage of false Positives = b/(b+d) x 100

Sensitivity

• It is a statistical index of diagnostic accuracy.

• Defined as the ability of a test to identify correctly all those who have the disease, i.e., true positive.

• A 90 percent sensitivity means that 90 percent of the diseased people screened by the test will give a “true positive” result and the remaining 10 percent a “false negative” result.

Specificity

• It is defined as the ability of a test to identify correctly those who do not have the disease, i.e., “ true negatives”.

• 90 % specificity means that 90 percent of the non-diseased persons will give “true negative” result, 10 % pf the non-diseased persons screened by the test will be wrongly classified as “diseased” when they are not

False negative

• The term false negative means that patients who actually have the disease are told that they do not have the disease.

• It amounts to giving them false reassurance.• A screening test which is very sensitive has

few false negative.• Lower the sensitivity, larger will be the

number of false negatives.

False Positive

• The term false positive means that patients who do not have the disease are told that they have.

• A screening test with a high spcificity will have few false positives.

Positive Predictive value

• proportion of disease individuals among those with positive test results or probability of the person having the disease when the test is positive

Negative predictive value

• proportion of individuals without disease among those negative test results or probability of the person not having the disease when the test is negative

Yield

Yield is the amount of previously unrecognized disease that is diagnosed as a result f the screening effort.

It depends upon the many factors such as sensitivity and specificity of the test, prevalence of disease, participants of individuals in the detection program.

High risk population are usually selected for screening, thus increasing the yield.

Screening Test Results by Diagnosis

Screening test results Diagnosis Total

Diseased Not diseased Positive a (True positive)

40b (False positive) 20

a+b60

Negative c (False negative) 100

d (True negative) 9840

c+d9940

Total a+c 140

b+d 9860

a+b+c+d10,000

Evaluation of screening test

a) Sensitivity (true positive)= 40/140 x 100 = 28.57%

b) Specificity (true negative) = 9840/9860 x 100 = 99.79%

c) False negative = 100/140 x 100 = 71.4 %

d) False positive = 20/9860 x 100 = 0.20 %

e) Predictive value of positive test = 40/60 x 100 = 66.67%

f) Predictive value of negative test

= 9840/9940 x 100 = 98.9%

The more prevalent a disease the more accurate will be the predictive value of a positive screening test .

The best diagnostic tests are those that yield few false positives and few false negatives: high sensitive and

high specific

Uses of sensitive test

 

1. A sensitive test should be choosen when there is an important penalty for missing a disease. The diseases which are dangerous but treatable e.g. tuberculosis, syphilis, breast cancer etc.

 2. Proportion of false positive is tolerable but not false negative

Uses of specific test

1. It is useful to confirm the diagnosis that has been suggested by other data.

2. Highly specific tests are particularly needed when false positive results can harm patient physically, emotionally, or financially.

Trade off between sensitivity and specificity

• One characteristic (e.g. sensitivity) can only be increased at the expense of the other (e.g. specificity).

• Simply, we can get the sum of sensitivity and specificity of each test and that test which has highest sum is selected for the diagnosis.

 • A figure plotting the true

positive rate (sensitivity) against the false positive rate is called the receiver operator characteristics of the test (ROC curve).

Sen

siti

vit

y (

%)

Tru

e p

osi

tiv

e ra

te

1 - s p ec if ic ity ( % )F als e p o s it iv e r a tes

2 0 4 0 6 0 8 0 1 0 0

2 0

4 0

6 0

8 0

1 0 0

Figure: ROC curve

Criteria for Assessing the Screening Test

 

Simplicity- a test should be simple to perform, and easy to interpret

 

Acceptability- since participation in screening is voluntary, a test should be acceptable to those undergoing it.

 

Accuracy- a test must give true measurement of the condition or symptom under investigation

 

Cost- the expense of test must be considered in relation to the benefits of detection of disease

 

Precision or repeatability- the tests should give consistent results in repeated examinations

 

Sensitivity- a test should be capable of giving positive findings when the person being screened has the disease being sought

 

Specificity- a test should be capable of giving negative findings when a person being screened does not have the disease being sought.

 

Reliability of screening test

• It is also known as repeatability. • Factors contribute to the variation between test

are: – Intra subject variation

• Many human characteristics vary because of the change in time of day even during a short period of time, and because of conditions under which certain tests are conducted (e.g. pp glucose )

– Inter-observer variation: variation between observers

– Overall percent agreement (OPA) = (a+d)/(a+b+c+d) x 100

Criteria for Instituting a Screening Program 

Disease • Serious • High prevalence of preclinical stage • Natural history understood • Long period between first signs and overt disease

 

Diagnostic tests• Sensitive and specific• Simple and cheap • Safe and acceptable • Reliable

 

Evaluation of Screening Programme

• Evaluation of screening test can be performed by– Randomized controlled trials– Uncontrolled trials ( cervical cancer screening )– Other methods ( case control and comparison

studies0

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